1. Field of the Invention
This invention relates to a rotary head type magnetic recording/reproducing unit to be used for a DAT or digital VTR machine and, more particularly, it relates to a rotary head type magnetic recording/reproducing unit provided with a control means for stabilizing envelope waveforms at the starting point or the ending point or both the starting and ending points of each envelope waveform during recording or reproducing operation.
2. Prior Art
FIG. 6 of the accompanying drawings shows a plan view of the rotary head section of a rotary head type magnetic recording/reproducing unit to be used for a DAT machine.
In a DAT machine, there are provided a pair of magnetic heads A and B arranged on a rotary drum 1a of a rotary head unit 1. When the rotary drum of a DAT machine has a diameter of 30mm, magnetic tape T is driven past the rotary drum while it is in frictional contact with the drum's periphery over a distance that corresponds to a central angle of 90.degree.. In practice, however, a tolerance of some 0.5 to 1.5.degree. is given to the both ends of the tape on the contact area of the drum's periphery in view of the accuracy of operation of the tape loading and driving mechanism. FIG. 7 is a plan view of a magnetic tape schematically illustrating how signals are recorded on a tape. In FIG. 7, the hatched area is the area where signals are recorded in one recording operation (or the area that corresponds to the central angle of 90.degree. plus said tolerances).
FIG. 8(a) illustrates envelope waveforms for signals recorded on the tape and FIG. 8(b) shows the timing switch-over from the magnetic head A to the magnetic head B. With a rotary head type magnetic recording/reproducing unit of a DAT machine as illustrated in FIG. 6, since each of the magnetic heads A and B covers an operating range 90.degree., the recorded signals show intermittently arranged waveforms where a void angle of 90.degree. takes place for every 180.degree. rotation of the drum. Conventionally, the switch-over from the head A to the head B or vice versa is conducted in each void angle as seen from FIG. 8(b).
As the switch-over from the head A to the head B or vice versa takes place in a conventional DAT machine at a timing as illustrated in FIG. 8(b) and described above, the head A, for example, carries out a recording operation while the tape is running for a distance that corresponds to a central angle of 90.degree. and then, after an intermission of 90.degree., the head B takes the role of recording signals all the way as the tape is running for a same distance that also corresponds to a central angle of 90.degree..
With a known rotary head type magnetic recording/reproducing unit as described above, however, there arises a problem of distorted or deformed envelope waveforms at every start of recording operation as seen from FIG. 8(a) because of unstable contact between the magnetic head and the tape that can occur in the initial stages contact where the tape comes to touch and frictionally move on the magnetic head. A similar problem occurs when the tape leaves the magnetic head or at the ending point of an envelope waveform for an obvious reason. Distorted envelope waveforms can very often result in inaccurate recording or reproduction of signals and clock synchronization of signals can be severely affected particularly when the envelope waveform is distorted in the initial stages.
There have been proposed measures that address the problem of distortion or deformation of envelope waveforms. For instance, Japanese Patent Disclosure, Tokkou Shou, No. 59-14812 teaches a method of suppressing distortion of envelope waveforms by eccentrically arranging the gaps between the magnetic heads. However, since such a method requires specifically designed magnetic heads, the overall cost of a DAT machine comprising such heads can be significantly higher than an ordinary DAT machine, although it is not completely free from the problem of distorted envelope waveforms.
It is therefore an object of the present invention to solve the above problem by providing a rotary head type magnetic recording/reproducing unit which is free from the problem of distorted envelope waveforms at the starting point or the ending point or both the starting and ending points of each envelop waveform during recording or reproducing operation by simply specifying the central angle of contact of each of the magnetic heads relative to the recording tape and the timing of head switch-over.